Ink jet printing is a well known method of printing using a liquid ink ejected in small droplets from a small orifice. Liquid ink can be water based or based on other solvents. Liquid ink can also be generated by melting of a solid, wax-like ink. The colorant in the ink can be a dye or a pigment. While the current invention applies to all methods of ink jet printing and to all types of ink, its biggest benefit is obtained when using water based inks. The term "colorant" is used here in a generic sense and covers any component of the ink which remains after the carrier liquid evaporated. An ink component can be a colorant even if its function in the ink is not as a color. For example, when the ink is used as an etch-resist or when it is used to make a printing plate the colorant may be transparent and colorless.
All inks used in ink jet printing today wet the substrate they are deposited on. This wetting is key to adhesion and durability of the finished product. The terms "wetting" and "non-wetting" refer to the appearance of the droplet on the substrate before it dries or solidifies. FIG. 1. Shows the prior art, in which a droplet 1 is ejected from a nozzle 2 onto a substrate, normally paper, 3. After a few milliseconds of bouncing the droplet starts wetting the substrate as shown in FIG. 1-d. The wetting manifests itself as lowering of the contact angle .theta. to well below 90.degree.. At the same time some absorption into the substrate takes place. The dry droplet, FIG. 1-e, has some colorant absorbed into the substrate. It is known that heating the printed substrate can increase ink adhesion and durability (see for example U.S. Pat. No. 4,308,542).
Wetting is an essential part of all prior art ink jet applications. When a material which may interfere with wetting is used to coat the paper, the material has to be treated to become highly porous or wettable by some other means. Making material highly porous promotes wetting as increasing the surface area of a material increases the surface energy, and increasing the surface energy increases wetting. Some materials which will not be wet by a liquid when applied in a continuous and smooth layer will wet well when made porous. This is the basis of U.S. Pat. No. 5,405,678 which uses a hydrophobic latex to improve paper surface but does not allow latex particles to coalesce (fuse together). The significance of leaving the surface porous is clearly stated in U.S. Pat. No. 5,405,678 (page 6, lines 13-30). This patent also recommends mixing a very hydrophilic material, such as aluminum silicate or activated clay, to promote wetting. U.S. Pat. No. 5,099,256, issued to Anderson, discloses a technique that substantially reduces wetting of a paper recording medium by ink droplets. Anderson employs an intermediate non-wettable drum surface on which ink droplets are sprayed and then dehydrated with heat before they are transferred to the final paper recording medium. The Anderson invention helps to reduce the amount of wetting on a paper recording medium and thereby reduces the dot size, dor irregularity and color to color bleeding, however it requires a more complex system with an intermediate transfer drum. Such silicone coated drums have low durability due to the softness of the silicone and the need to maintain a very fine texture on the surface. The current invention overcomes these problems. The major disadvantage of wetting is that it generates dot sizes which are too large for high quality printing, particularly in the highlight areas of pictures. The problem is more severe when wetting is followed by absorption into the paper fibers. This causes dots not only to grow but become irregular.
It is the main object of this invention to generate very small and well defined dots using all conventional ink jet printing processes and in particular when using water based inks. A second object of the invention is to generate printing plates for other methods of printing, such as lithographic printing plates and flexographic printing plates. A third object of the invention is to use the ability to create very fine dots to deposit directly chemically resistant coatings and in particular etch resists (also known by the generic name "resist") to act as masks during etching. These and further objects will become clear from the following description of the invention.